Multi beam charged particle device

ABSTRACT

The present invention provides an improved column for a charged particle beam device. The column comprises an aperture plate having multiple apertures to produce multiple beams of charged particles and a deflector to influence the beams of charged particles so that each beam appears to come from a different source. Furthermore, an objective lens is used in order to focus the charged-particle beams onto the specimen. Due to the deflector, multiple images of the source are created on the surface of the specimen whereby all the images can be used for parallel data acquisition. Accordingly, the speed of data acquisition is increased. With regard to the focusing properties of the objective lens, the beams of charged particles can basically be treated as independent particle beams which do not negatively affect each other. Accordingly, each beam basically provides the same resolution as the beam of a conventional charged particle beam device.

This is a National stage entry under 35 U.S.C. § 371 of Application No.PCT/EP01/04787 filed Apr. 27, 2001; the disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to an apparatus for the examination of specimenwith charged particles. In particular, this invention relates to anapparatus for the examination of specimen with multiple beams of chargedparticles.

BACKGROUND OF THE INVENTION

Modern semiconductor technology is highly dependent on an accuratecontrol of the various processes used during the production ofintegrated circuits. Accordingly, the wafers have to be inspectedrepeatedly in order to localize problems as early as possible.Furthermore, a mask or reticle should also be inspected before itsactual use during wafer processing in order to make sure that the maskaccurately defines the desired pattern. This is done because any defectsin the mask pattern will be transferred to the substrate (e.g., wafer)during its use in microlithography. However, the inspection of wafers ormasks for defects requires the examination of the whole wafer or maskarea. Especially, the inspection of wafers during their fabricationrequires the examination of the whole wafer area in such a short timethat production throughput is not limited by the inspection process.

Scanning electron microscopes (SEM) have been used to inspect wafers todetect pattern defects. Thereby, the surface of the wafer is scannedusing a single finely drawn electron beam. When the electron beam hitsthe wafer, secondary electrons are generated and measured. A patterndefect at a location on the wafer is detected by comparing an intensitysignal of the secondary electrons to, for example, a reference signalcorresponding to the same location on the pattern. However, because onlyone very narrow electron beam is used for scanning, a long time isrequired to scan the entire surface of the wafer. Accordingly, it is notfeasible to use a conventional (single-beam) Scanning ElectronMicroscope (SEM) for wafer inspection, since this approach does notprovide the required throughput. Therefore, high speed wafer inspectionis presently carried out by means of light optical techniques.

In order to perform this task using electron microscopic techniquesseveral approaches have been suggested. One approach is based on theminiaturization of SEMs, so that several miniaturized SEMs (in the orderof ten to one hundred) are arranged in an array and each miniaturizedSEM examines a small portion of the complete sample surface. Anotherapproach makes use of fixed-beam surface electron microscopes whichimage a certain area of the sample simultaneously. These surfacemicroscopes can be classified by the excitation process of the electronsthat form the image at the detector: a) The Photoemission ElectronMicroscope (PEEM), where the electrons are created by illumination ofthe sample surface with UV light, synchrotron radiation, or X-rays andb) the so-called Low-energy Electron Microscope (LEEM), where, invarious modes of operation, the sample surface is illuminated withelectrons. In this case, the illuminating electrons have to be separatedfrom the imaging electrons by means of an additional electron opticalelement, for example, a beam separator in the form of a dipole magnet.However, both approaches have not yet been put into industrial practice.

Multi-beam electron projection systems are used to create patterns ofvariable shape on a substrate by switching on and off individual beamsas is described in document EP 0 508 151. The following remarks areparticularly relevant: First, as a projection system, it inherently doesnot produce an image of a sample and, therefore, does not comprise anobjective lens. Secondly, in the example to which we referred to above,a resulting electron beam, formed by the individual beams that are notblanked out, is scanned as a whole over the substrate.

Furthermore, SEMs using multiple charged particle beams have beensuggested in order to increase the throughput of data collectionprocess. For example, U.S. Pat. No. 5,892,224 describes an apparatus forinspecting masks and wafers used in microlithography. The apparatusaccording to U.S. Pat. No. 5,892,224 is adapted to irradiate multiplecharged particle beams simultaneously on respective measurement pointson the surface of a sample. However, the apparatus according to U.S.Pat. No. 5,892,224 is primarily designed for the inspection of masks anddoes not provide the resolution which is required to inspect theintricate features present on a semiconductor wafer.

In charged particle beam devices, such as a scanning electron microscope(SEM), the charged particle beam exhibits a typical aperture angle aswell as a typical angle of incidence in the order of several millirads.However, for many applications, it is desirable that the chargedparticle beam hits the sample surface under a much larger angle oftypically 5° to 10°, corresponding to 90 to 180 millirads. Stereoscopicvisualization is an example of such an application. Some applicationseven require tilt angles in excess of 15° or even 20°. In many cases,the additional information which is contained in stereo images isextremely valuable in order to control the quality of a productionprocess.

Thereby, a number of tilting mechanism can be used. In early solutions,an oblique angle of incidence was achieved by mechanically tilting thespecimen. However, apart from other drawbacks, mechanically tilting thespecimen takes a lot of time. An oblique angle of incidence may also beachieved by electrically tilting the charged particle beam. This can bedone by deflecting the beam so that either by the deflection alone or incombination with the focussing of the beam, an oblique angle ofincidence results. Thereby, the specimen can remain horizontal which isa significant advantage as far as the lateral coordinate registration isconcerned. Furthermore, electrical tilting is also much faster than itsmechanical counterpart. However, even though electrical tilting is inprincipal faster than its mechanical counterpart, additional alignmentprocedures are usually required when the beam is shifted electricallyfrom angle of incidence to another angle of incidence. These additionalalignment also require a considerable amount time. Therefore,stereoscopic visualization is not routinely done in the semiconductorindustry.

Accordingly, there is a need for a charged particle beam device whichprovides a sufficient resolution and which is able to increase the datacollection to such an extent that the device can also be applied to highspeed wafer inspection. Furthermore, there is a need for a chargedparticle beam device which is able to reduce the time that is needed toproduce a pair of stereo images.

SUMMARY OF THE INVENTION

The present invention provides an improved column for a charged particlebeam device. According to one aspect of the present invention, there isprovided a column for a charged particle beam device as specified inindependent claim 1. According to a further aspect of the presentinvention there is provided a column for a charged particle beam deviceas specified in independent claim 9. Further advantageous features,aspects and details of the invention are evident from the dependentclaims, the description and the accompanying drawings. The claims areintended to be understood as a first non-limiting approach of definingthe invention in general terms.

The present invention provides an improved column for a charged particlebeam device. The column comprises an aperture plate having multipleapertures to produce multiple beams of charged particles and a deflectorto influence the beams of charged particles so that each beam appears tocome from a different source. Furthermore, an objective lens is used inorder to focus the charged-particle beams onto the specimen. Due to thedeflector, multiple images of the source are created on the surface ofthe specimen whereby all the images can be used for parallel dataacquisition and/or for parallel modification of the specimen.Accordingly, the speed of data acquisition (modification) is increased.With regard to the focusing properties of the objective lens, the beamsof charged particles can basically be treated as independent particlebeams which do not negatively affect each other. Accordingly, each beambasically provides the same resolution as the beam a conventionalcharged particle beam device.

According to a further aspect of the present invention, an improvedcolumn for a charged particle beam device is provided which is capableof producing a stereoscopic image of the surface of specimen in a singlescan over the surface of specimen. The column comprises an apertureplate having multiple apertures to produce multiple beams of chargedparticles and a deflector to influence the beams of charged particles sothat each beam appears to come from a different source and that eachbeam passes through the objective lens along an off-axis path. Due tothe off-axis path through the objective lens, the charged particle beamsare tilted and hit the specimen under oblique angle of incidences.Furthermore, each beam is tilted into a different direction so that theimages of two of the beams are sufficient to produce a stereoscopicimage of the surface of specimen. Since there no need for any additionalalignments of beams with regard to the column, the time that is requiredto produce a stereoscopic image is reduced considerably.

BRIEF DESCRIPTION OF THE DRAWINGS

Some of the above indicated and other more detailed aspects of theinvention will be described in the following description and partiallyillustrated with reference to the figures. Therein:

FIG. 1 shows schematically a column according to a first embodimentaccording to the present invention,

FIG. 2 is an enlarged view showing the objective lens of the embodimentof FIG. 1.

FIG. 3 is an enlarged view of FIG. 2.

FIG. 4 shows schematically the aperture plate as used in the columnshown in FIG. 1,

FIG. 5 shows schematically a top view on the cylinder lens as used inthe column shown in FIG. 1,

FIG. 6 shows schematically a bottom view on the detector as used in thecolumn shown in FIG. 1,

FIG. 7 shows schematically an column according to a second embodimentaccording to the present invention

FIG. 8 shows schematically the aperture plate as used in the columnshown in FIG. 7,

FIG. 9 shows schematically a bottom view on the detector used in thecolumn shown in FIG. 7,

FIG. 10 shows schematically a column according to a third embodimentaccording to the present invention,

FIG. 11 shows schematically the aperture plate as used in the columnshown in FIG. 10,

FIG. 12 shows schematically a top view on the deflectors as used in thecolumn shown in FIG. 10,

FIG. 13 shows schematically a bottom view on the detector as used in thecolumn shown in FIG. 10,

FIG. 14 shows schematically a column according to a fourth embodimentaccording to the present invention,

FIG. 15 is an enlarged view showing the objective lens of the embodimentof FIG. 14,

FIG. 16 shows schematically a column according to a fifth embodimentaccording to the present invention,

FIG. 17 shows schematically a top view on the compensation unit as usedin the column shown in FIG. 16,

FIG. 18 shows schematically a column according to a further embodimentaccording to the present invention,

FIGS. 19, 20 show schematically a top view and a side view on thecompensation unit as used in the column shown in FIG. 18,

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment according to the invention is shown schematically in FIG.1. The column 1 for a charged particle beam device comprises a chargedparticle source 2 which emits a beam 4 of charged particles. In electronbeam devices, electron sources such as Tungsten-Hairpin guns,Lanthanum-Hexaboride Guns, Field emission guns etc. can be used. Theinvention, however, is not limited to electron sources; it can be usedtogether with all kinds of charged particle sources. The chargedparticles are accelerated by an accelerating voltage supplied to thecharged particle source 2.

After leaving the charged particle source 2, the charged particle beam 4passes through an aperture plate 5 having multiple apertures 5A-5H whichare situated along a ring on the aperture plate 5. By passing throughthe aperture plate 5, multiple charged particle beams 4A-4H are created.Thereafter, a deflector 6 is used to influence the beams of chargedparticles 4A-4H so that each beam 4A-4H appears to come from a differentsource. In the present embodiment the deflector 6 has the form of acylinder lens comprising two cylinder electrodes 6A and 6B which are setto appropriate potentials. For example, the potential difference betweenthe inner cylinder electrode 6A and the outer cylinder electrode 6B isset to +300 Volts so that the beams of charged particles 4A-4H areattracted towards the inner cylinder electrode 6A.

The charged particle beams 4A-4H then pass the detector 9 which is usedto detect those particles that come from the specimen 8 in order toproduce multiple images of the specimen 8. The detector 9 is divided inmultiple segments 9A-9H corresponding to the multiple beams of chargedparticles 4A-4H. Thereby, each segment 9A-9H of the detector is capableof detecting particles that come from the specimen 8 independent of allthe other segments of the detector 9. Furthermore, the detector 9comprises multiple openings to let the beams of charged particles 4A-4Hpass through.

The detector 9 is followed by the scanning coils 12 which are used tomove the charged particle beams 4A-4H in a raster over the surface ofthe specimen 8. After the scanning coils 12, the charged particle beams4A-4H enter the objective lens 10 that focuses the charged particlebeams 4A-4H onto the specimen 8. The objective lens 10 not only focusesthe charged particle beams 4A-4H but also rotates the charged particlebeams 4A-4H. However, this effect is not shown because it is difficultto depict in a two-dimensional drawing and because the skilled person iswell aware of this additional effect. Due to the combined effects of thedeflector 6 and the objective lens 10, multiple spots (images of theparticle source 2), each corresponding to one of the charged particlebeams 4A-4H, are created on the specimen 8. Without the deflector 6, theobjective lens 10 would focus the charged particle beams 4A-4H into asingle spot on the specimen 8.

When the particles of beams 4A-4H strike the surface of the specimen 8,they undergo a series of complex interactions with the nuclei andelectrons of the atoms of the specimen. The interactions produce avariety of secondary products, such as electrons of different energy, Xrays, heat, and light. Many of these secondary products are used toproduce the images of the sample and to collect additional data from it.A secondary product of major importance to examination or the imageformation of specimens are secondary electrons that escape from thespecimen 8 at a variety of angles with relatively low energy (3 to 50eV). The secondary electrons are drawn through the objective lens 10,reach the detector 9, and are then measured. Thereby, the parameters ofthe objective lens 10 are chosen in such a manner that the secondaryelectrons and/or backscattered particles that come from specimen arefocused onto the detector 9. Accordingly, each spot on the specimenleads to a corresponding spot on the detector. Thereby, the segments9A-9H of the detector 9 are selected so that each spot basically hitsthe corresponding segment 9A-9H in its center.

By scanning the charged particle beams 4A-4H over the specimen anddisplaying/recording the output of the detector 9, multiple independentimages of the surface of the specimen 8 are formed. Each image containsinformation about a different portion of the surface of the specimen.Accordingly, the speed of the data acquisition is increased by a factor8 with regard to the conventional single beam case. The specimen 8 issupported on a stage 7 (specimen support) which is moveable horizontallyin all directions, in order to allow the charged particle beams 4A-4H toreach the target areas on the specimen which are to be examined.

In order to improve the performance of the system, the embodiment shownin FIG. 1 contains an objective lens 10 which is a combination of amagnetic lens 10A and an electrostatic lens 10B. Accordingly, theobjective lens 10 is a compound magnetic-electrostatic lens. Preferably,the electrostatic part of the compound magnetic-electrostatic lens 10 isan electrostatic retarding lens 10B. Using such a compoundmagnetic-electrostatic lens 10 yields superior resolution at lowacceleration energies, such as a few hundred electron volts in the caseof a SEM. Such low acceleration energies are desirable, especially inmodem semiconductor industry, to avoid charging and/or damaging ofradiation sensitive specimens. However, the benefits of the presentinvention are also achieved if only a magnetic lens or only anelectrostatic lens is used.

FIGS. 2 and 3 show enlarged views on the compound magnetic-electrostaticlens 10 and the specimen 8 as shown in FIG. 1. To achieve a small focallength, the magnetic flux generated by a current through an excitationcoil is conducted through pole pieces and is concentrated into a smallregion along the optical axis of the magnetic lens. The magnetic fieldis rotationally symmetric around the optical axis and reaches itsmaximum value in the pole gap between the upper and the lower polepiece. In addition to the magnetic lens 10A, the embodiment shown inFIGS. 1 to 3 contains an electrostatic retarding lens which is situatedclose to magnetic lens 10A. The electrostatic retarding lens 10B has twoelectrodes held at different potentials. In the illustrated embodiment,one of the two electrodes is formed by a cylindrical beam tube 14 whichis arranged within the magnetic lens 10A along the optical axis Thesecond electrode of the electrostatic retarding lens 10B is a metalliccup provided below the magnetic lens 10A. In operation of the system,the first electrode is usually held at high positive potential, forexample 8 kV, where as the second electrode is held at lower positivepotential, for example 3 kV, so that the electrons are decelerated inthe corresponding electrostatic field from a first energy to a lowersecond energy.

In the example shown in FIGS. 2 and 3, the specimen 8 is held at groundpotential. Accordingly, there is a further electrostatic retarding fieldbetween the metallic cup and the specimen 8. However, the surface of thespecimen need not be grounded. The electric potential on the surface ofthe specimen may also be adjusted by applying a voltage to the specimen.A voltage can be applied to a wafer, for example, in order to obtainvoltage contrast imaging which is used to detect shorts in a circuit. Aslong as the potential of the metallic cup is higher than the potentialon the surface of the specimen, an electrostatic retarding field isproduced. Furthermore, as long as the potential of the metallic cup ishigher than the potential on the surface of the specimen, the secondaryelectrons (backscattered particles) are drawn into the objective lens 10and reach the detector 9.

FIG. 4 shows schematically a top view on the aperture plate 5 as used inthe column shown in FIG. 1. The aperture plate S comprises eightapertures 5A-5H which are situated along a ring parallel to the edge ofthe aperture plate 5. The aperture plate 5 is made of conductingmaterial in order to avoid any charging effects. The size of theapertures 5A-5H is selected so that a predetermined current can beprovided.

FIG. 5 shows schematically a top view on the deflector 6 as used in thecolumn shown in FIG. 1. The deflector 6 has the form of a cylinder lenscomprising two cylinder electrodes 6A and 6B which are set toappropriate potentials. In order to attract the charged particle beams4A-4H towards the inner cylinder electrode 6A, a potential differencebetween the inner cylinder electrode 6A and the outer cylinder electrode6B is provided. The appropriate potential difference between the innercylinder electrode 6A and the outer cylinder electrode 6B depends on anumber of different parameters, for example the energy of the chargedparticles inside the deflector 6 or the desired distance between twoadjacent beams on the surface of the specimen. Usually, the potentialdifference lies in the range of 100 to 500 Volts.

FIG. 6 shows schematically a bottom view on the detector 9 as used inthe column shown in FIG. 1. The detector 9 is divided in multiplesegments 9A-9H each having the form of a piece of cake and eachcorresponding to one of charged particle beams 4A-4H. Thereby, eachsegment 9A-9H of the detector is capable of detecting particles thatcome from the specimen 8 independent of all the other segments of thedetector 9. Furthermore, detector 9 comprises multiple openings 11A-11Hto let the beams of charged particles 4A-4H pass through. The detector 9is orientated in such a manner so that each spot on the specimen isfocused to the center of the corresponding segment 9A-9H.

FIG. 7 shows schematically a column according to a second embodiment ofthe present invention. This embodiment is similar to that of FIG. 1,except for the following; The column comprises a beam selector 16located right after the particle source 2. The beam selector 16comprises a first deflector 16A, aperture plate 16B, a second deflector16C, and a third deflector 16D. The aperture plate 16B comprises twoapertures having two different diameters. After the beam of chargedparticles leaves the particle source 2, the first deflector 16A deflectsthe charged particle beam 4 towards one the two aperture in the apertureplate I&B. Thereafter, the second and the third deflector bring thecharged particle beam 4 back to its way along the optical axis of thecolumn.

The aperture plate 15 used in FIG. 7 comprises, in addition to theapertures 15A-15H situated along a ring parallel to the edge of theaperture plate, a further aperture 151 situated in the center of theaperture plate (see FIG. 8). Accordingly, a further beam of chargedparticles 4I is produced along the optical axis of the column. Thedetector 19 also contains an additional opening 11I in order to let thebeam 4I of charged particles pass through the detector 19 (see FIG. 9).

If the charged particle beam is directed to the smaller aperture in theaperture plate 16B, the resulting beam 4 will be so small that theapertures 15A-15H situated along the ring parallel to the edge of theaperture plate are not illuminated by charged particle beam 4.Accordingly, only charged particle beam 4I is formed. In the following,the charged particle beam 4I passes through the center of the innerelectrode 6A without being affected by the field of the cylinder lens 6.Furthermore, the charged particle beam 4I passes through the detector 19and is focused onto the specimen. Since there are no further chargedparticle beams, the column basically functions as a conventional singlebeam device.

If the charged particle beam is directed to the larger aperture in theaperture plate 16B, the resulting beam 4 will be large enough so thatthe apertures 15A-15H situated along the ring parallel to the edge ofthe aperture plate are illuminated by charged particle beam 4.Accordingly, all charged particle beams 4A-4I are formed. In this mode,the column shown in FIG. 7 basically operates as the column shown inFIG. 1. Since, in this mode, the parameters of the objective lens 10 arechosen in such a manner that the secondary electrons and/orbackscattered particles that come from specimen are focused onto thedetector 19, the secondary electrons corresponding to the center beam 4Iare focused on the center of the detector 19. Accordingly, theseelectrons are not measured by the detector 19.

Due to the usage of the beam selector 16, the operator may switch easilybetween two modes of operation. Accordingly, the device shown in FIG. 7can be adapted in a fast and efficient manner to the specificmeasurement needs.

FIG. 10 shows schematically a column according to a third embodiment ofthe present invention. This embodiment is also similar to that of FIG.1, except for the following; After leaving the charged particle source2, the charged particle beam 4 passes through the aperture plate 25having four apertures 25A-25D which are situated at equal distance alonga ring on the aperture plate 25. By passing through the aperture plate25, four charged particle beams 4A-4D are created. Thereafter, thedeflectors 26A-26D are used to influence the beams of charged particles4A-4D so that each beam 4A-4D appears to come from a different source.The deflectors 26A-26D influence each of the beams 4A-4D individuallywhich leads to a better control of the properties of each individualbeam.

The charged particle beams 4A-4D then pass the detector 29 which is usedto detect those particles that come from the specimen 8 in order toproduce multiple images of the specimen 8. The detector 29 is divided inmultiple segments 29A-29D corresponding to the multiple beams of chargedparticles 4A-4D (see FIG. 13). Thereby, each segment 29A-29D of thedetector is capable of detecting particles that come from the specimen 8independent of all the other segments of the detector 29. Furthermore,detector 29 comprises multiple openings to let the beams of chargedparticles 4A-4D pass through.

FIG. 12 shows schematically a top view on the deflectors 26A-26D as usedin the column shown in FIG. 10. Thereby, each deflector 26A-26Dcomprises eight electrodes. Each deflector 26A-26D generates staticdeflecting fields for correction of the beam paths through the objectivelens and for positioning the beams at the specimen. Furthermore, thedeflectors 26A-26D can be used for compensation of the aberrationsarising from a deviation of the objective lens from the axial symmetry.

FIG. 14 shows schematically a column according to a third embodiment ofthe present invention. This embodiment is also similar to that of FIG.1, except for the following; After leaving the charged particle source 2the charged particle beam 4 passes through the aperture plate 25 havingfour apertures 25A-25D which are situated at equal distance along a ringon the aperture plate 25. By passing through the aperture plate 25, fourcharged particle beams 4A-4D are created. Thereafter, the deflector 6 isused to influence the beams of charged particles 4A-4D so that each beam4A-4D appears to come from a different source. Furthermore, thedeflector 6 is used to influence the beams of charged particles 4A-4D sothat each beam 4A-4D traverses the objective lens 10 along an off-axispath. Due to the off-axis path of the charged particle beams 4A-4D, thebeams are tilted and hit the specimen under oblique angles of incidence.

FIG. 15 shows an enlarged view on the compound magnetic-electrostaticlens 10 and the specimen 8 as shown in FIG. 14. Thereby, the beam 4Ahits the specimen 8 under an oblique angle of incidence −θ, as measuredwith regard to an axis normal to the surface of the specimen.Furthermore, the beam 4C hits the specimen 8 under an oblique angle ofincidence +θ. The electron beams 4A and 4C do not hit the specimen atthe same spot but are displaced from each other by a distance D. Byscanning the two beams 4A and 4C over the surface of the specimen theimages from the two beams 4A and 4C are recorded. Based on the fact thatbeams hit the specimen under different angles of incidence, the imagesof the two beams are sufficient to produce a stereoscopic image of thesurface of specimen.

As outlined with regard to FIG. 1, the electrostatic retarding lens 10Bhas two electrodes held at different potentials. In the illustratedembodiment, one of the two electrodes is formed by a cylindrical beamtube 14 which is arranged within the magnetic lens 10A along the opticalaxis. The second electrode of the electrostatic retarding lens 10B is ametallic cup provided below the magnetic lens 10A. In operation of thesystem, the first electrode is usually held at high positive potential,for example 8 kV, where as the second electrode is held at lowerpositive potential, for example 3 kV, so that the electrons aredecelerated in the corresponding electrostatic field from a first energyto a lower second energy.

In the example shown in FIG. 15, the specimen 8 is held at groundpotential. Accordingly, there is a further electrostatic retarding fieldbetween the metallic cup and the specimen 8. Due to the electrostaticretarding field between the metallic cup and the specimen 8, the initialtilt of the charged particle beams 4A-4D caused by the off-axis path ofthe charged particle beams 4A-4D is enhanced leading to increased anglesof incidence. The surface of the specimen need not be grounded. Theelectric potential on the surface of the specimen may also be adjustedby applying a voltage to the specimen. A voltage can be applied to awafer, for example, in order to obtain voltage contrast imaging which isused to detect shorts in a circuit. As long as the potential of themetallic cup is higher than the potential on the surface of the specimen8, an electrostatic retarding field is produced.

By scanning the four beams 4A-4D over the surface of the specimen,stereoscopic images of the surface can be made in a single scan.Accordingly, stereoscopic images of a specimen can be produced in a fastand reliable manner without the need for any additional alignments.Therefore, the additional information, which is contained in stereoimages and which is extremely helpful in many cases, can be accessedwithout causing any additional costs and without causing any additionaltime delays.

The embodiment shown in FIG. 14 uses the off-axis path of the beamsthrough the objective lens 10 in order to tilt the beams 4A-4D. However,the off-axis path of the beams through the objective lens 10 gives riseto chromatic aberrations. The charged particles, usually electron, inthe beams are not monochromatic, but are emitted with slightly differentenergies. For example, in a thermionic electron gun, the energy spread(i.e., the full width at half maximum of the electron energydistribution) is on the order of ΔE=2.5 kT_(c), where T_(c) is thetemperature of the cathode tip and k is Boltzmann's constant. Thisenergy spread is further increased by the Boersch effect arising fromspace-charge oscillations near a crossover, so that thermionic tungstencathodes show an energy spread of ΔE=1-3 eV, while for LaB₆ cathodes thevalue is ΔE=0.5-2 eV. Field emission guns usually have a lower energyspread due to the smaller cathode temperatures of the order ofΔE=0.2-0.4 eV (L. Reimer, Scanning Electron Microscopy, Springer, 1985).

In order to decrease the chromatic aberrations, FIG. 16 shows aschematic diagram of an apparatus according to a further embodiment ofthe present invention. This embodiment is similar to that of FIG. 10,except for the following. The deflector 6 has been replaced by anintegrated unit 17 comprising a deflector 17A and a coil arrangement 17B(see FIG. 17). The deflector 17A and the coil arrangement 17B generatecrossed electrostatic and magnetic deflection fields which disperse thebeams 4A-4D of charged particles. By dispersing the beams of chargedparticles a second chromatic aberration of substantially the same kindand magnitude but opposite direction as the chromatic aberration causedoff-axis path of the beams through the objective lens can be produced.Accordingly, the chromatic aberration caused by the off-axis path can becompensated in the plane of the specimen surface.

FIG. 17 shows schematically a top view on the unit 17 as used in thecolumn shown in FIG. 16. The unit 17 comprises a deflector 17A havingthe form of a cylinder lens. The cylinder deflector 17A comprises twocylinder electrodes which are set to appropriate potentials. In order toattract the charged particle beams 4A-4D towards the inner cylinderelectrode, a potential difference between the inner cylinder electrodeand the outer cylinder electrode is provided. Thereby, the potentialdifference between the inner cylinder electrode and the outer cylinderelectrode is chosen in such a manner that the beam pass through theobjective lens along an off-axis path and hits the specimen underoblique angles of incidence. The appropriate potential differencebetween the inner cylinder electrode 6A and the outer cylinder electrode6B depends on a number different parameters; for example, the energy ofthe charged particles inside the deflector 6, the desired distancebetween two adjacent beams on the surface of the specimen, and theangles of incidence on the surface of the specimen. Usually, thepotential difference lies in the range of 100 to 500 Volts.

In addition to the cylinder deflector 17A, the unit 17 comprises a coilarrangement 17B which is used to generate a magnetic field withincylinder deflector 17A. Thereby, the magnetic field is essentiallyperpendicular to the electric filed generated by the inner cylinderelectrode and the outer cylinder electrode. The integrated unit 17 actslike a Wien filter, in which the electric field E and the magnetic fieldB generate an electric and a magnetic force on the charged particles,F_(el)=qE, and F_(mag)=q(v×B), wherein q=−e is the electron charge. Ifthe electric and magnetic field are perpendicular to each other and tothe velocity of the charged particle, the electric and magnetic forcesare in opposite directions. For particles with a certain velocity,ν=|E|/|B|, the net force is zero, and they pass the filter unaffected.Particles with a different speed experience a net forceF=|F_(el)−F_(mag)| and are deflected by the unit 17. In effect, thebeams of charged particles with a finite energy spread passing the unit17 are dispersed, as particles with different energies are deflected bydifferent amounts.

The dispersion leads to an at least partial compensation of thechromatic aberration caused by the off-axis pass through the objectivelens. The embodiment shown in FIG. 16 has thus the advantage that largeangles of incidence on the sample surface can be provided withoutreduction in resolution arising from large chromatic aberrations.

FIG. 18 shows schematically a column according to a still furtherembodiment according to the present invention. This embodiment issimilar to that of FIG. 16, except for the following; After passingthrough the aperture plate 25 four charged particle beams 4A-4D enterthe compensation units 36A-36D. The compensation units 36A-36D are againused to influence the beams of charged particles 4A-4D so that each beam4A-4D appears to come from a different source. Furthermore, thecompensation units 36A-36D are used to influence the beams of chargedparticles 4A-4D so that each beam 4A-4D traverses the objective lens 10along an off-axis path. The compensation units 36A-36D influence each ofthe beams 4A-4D individually which leads to a better control of theproperties of each individual beam

FIGS. 19 and 20 show the compensation unit 36A used in FIG. 18. Thecompensation unit 36A forms a Wien filter having an electrostatic andmagnetic quadrupole (4-pole). The quadrupole comprises four pole pieces42 and four electrodes 44. The electrodes and pole pieces are arrangedin a plane perpendicular to the path of the charged particles. As bestshown in FIG. 19, the electrodes and the pole pieces are each placedalong the circumference of a circle, spaced by an angle of π/2. Sincethe pole pieces and the corresponding electrodes have the same length(FIG. 20) and almost the same radius (FIG. 19), the resulting electricand magnetic field distributions are very similar leading to a goodcompensation of the electric and magnetic forces for the electrons withpredetermined energy in any point along the optical axis of the Wienfilter.

Using such a quadrupole arrangement, magnetic and electrostatic fieldscan be adjusted to deflect in an arbitrary direction in the planeperpendicular to the optical axis. Thereby, a compensation can beachieved for any direction of the deflecting action.

1. A column for a charged particle beam device, which is used to examineand/or modify a specimen, said column comprising: a) a source ofcharged-particles, b) an aperture plate having at least two apertures toproduce at least two beams of charged particles, c) at least onedeflector to influence the beams of charged particles so that each beamappears to come from a different source, d) at least one detector formeasuring secondary particles and/or backscattered particles coming fromthe specimen, and e) an objective lens for focusing the charged-particlebeams onto the specimen.
 2. The column according to claim 1, whereinsaid deflector comprises concentric cylinder electrodes.
 3. The columnaccording to claim 1, wherein a deflector is provided for eachindividual beam.
 4. The column according to claim 3, wherein thedeflectors are electrostatic multipoles, preferably selected from thegroup consisting of electrostatic dipole, quadrupole, hexapole andoctupole.
 5. The column according to any of preceding claims, whereinthe detector is positioned before the objective lens and comprisesmultiple openings to let the beams of charged particles pass through. 6.The column according to any of preceding claims, wherein the detector issubdivided into multiple segments corresponding to the multiple beams ofcharged particles.
 7. The column according to any of preceding claims,wherein the objective lens is adapted to focus the secondary particlesand/or backscattered particles onto the detector.
 8. The columnaccording to any of preceding claims, wherein said objective lenscomprises a magnetic lens and an electrostatic lens.
 9. The columnaccording to claim 8, wherein a first electrode and means for applying afirst potential to said first electrode are provided and wherein asecond electrode and means for applying a second potential to saidsecond electrode are provided to generate an electrical field in saidelectrostatic lens so that the particle beams in said electrical fieldare decelerated from a first energy to a second lower energy.
 10. Thecolumn according to any of preceding claims, wherein said apertures insaid aperture plate are arranged along at least one ring.
 11. The columnaccording to any of preceding claims, wherein the column comprises abeam selector for selecting the number of charged particle beams used toexamine the specimen.
 12. A column for a charged particle beam device,which is used to examine a specimen, said column comprising: a) a sourceof charged-particles, b) an aperture plate having at least two aperturesto produce at least two beams of charged particles, c) an objective lensfor focusing the charged-particle beams onto the specimen, d) at leastone deflector to influence the beams of charged particles so that eachbeam appears to come from a different source and so that by the combinedaction of the deflector and the objective lens the beams are tilted andhit the specimen with predetermined angles of incidence, and e) at leastone detector for measuring secondary particles and/or back-scatteredparticles coming from the specimen.
 13. The column according to claim12, wherein said deflector comprises concentric cylinder electrodes. 14.The column according to claim 12, wherein a deflector is provided foreach individual beam.
 15. The column according to claim 14, wherein thedeflectors are electrostatic multipoles, preferably selected from thegroup consisting of electrostatic dipole, quadrupole, hexapole andoctupole.
 16. The column according to one of claims 12 to 15, whereinthe detector is positioned before the objective lens and comprisesmultiple openings to let the beams of charged particles pass through.17. The column according to one of claims 12 to 16, wherein the detectoris subdivided into multiple segments corresponding to the multiple beamsof charged particles.
 18. The column according to one of claims 12 to17, wherein the objective lens is adapted to focus the secondaryparticles and/or backscattered particles onto the detector.
 19. Thecolumn according to one of claims 12 to 18, wherein said objective lenscomprises a magnetic lens and an electrostatic lens.
 20. The columnaccording to claim 19, wherein a first electrode and means for applyinga first potential to said first electrode are provided and wherein asecond electrode and means for applying a second potential to saidsecond electrode are provided to generate an electrical field in saidelectrostatic lens so that the particle beams in said electrical fieldare decelerated from a first energy to a second lower energy.
 21. Thecolumn according to one of claims 12 to 20, wherein said apertures insaid aperture plate are arranged along at least one ring.
 22. The columnaccording to one of claims 12 to 21, wherein the column comprises a beamselector for selecting the number of charged particle beams used toexamine the specimen.
 23. The column according to one of claims 12 to22, wherein the column comprises a least one compensation unit adaptedto disperse the beams of charged particles, thereby substantiallycompensating chromatic aberration in the plane of the specimen surface.24. The column according to claim 23, wherein the compensation unitcomprises means for generating crossed electrostatic and magneticdeflection fields.
 25. The column according to one of claim 23 or 24,wherein the compensation unit and the deflector are integrated into oneunit.
 26. The column according to claim 25, wherein the compensationunit comprises concentric cylinder electrodes and a coil arrangement.27. The column according to claim 25, wherein the compensation unit isan electrostatic and magnetic multipole, preferably selected from thegroup consisting of electrostatic and magnetic dipole, quadrupole,hexapole and octupole.
 28. The column according to one of claims 12 to27, wherein the column is adapted to provide angles of incidence between2° and 15°, preferable between 3° and 10°.